Maximal active movements of the mandible in the vertical and the horizontal plane were measured in 147 boys and 155 girls. with an age of 13, 15, 17 years respectively. The studied persons had no pain or severe symptoms of dysfunction of the masticatory system and the method used in this paper was devised by Agerberg, and the numerical calculations were performed at the Dept. of Medical Engineering of Seoul National University Hospital. The obtained results were as follows : 1. The mean values of maximal opening in 13 year-old boys and girls were 58.5 mm,58.9 mm respectively, and the mean values of boys were increased with age. 2. The mean values of maximal lateral movement to the right in 13-year-old boys and girls were 9.1 mm, 8.3 mm respectively, and the mean values to the left were 9.3 mm, 8.7 mm resprctively. The mean values were not increased with age in both. 3. The mean values of maximal protrusion in 13-year-old boys and girls were 8.7 mm, 8.1 mm respectivly, and in all ages the mean values of boys were larger than those of girls. 4. The range of maximal mandibular movements in teen-agers with 95% probability prediction ellipses were presented.